a) Field of the Invention
The invention is directed to an arrangement for generating EUV radiation based on electrically triggered gas discharges in which a vacuum chamber is provided for the generation of radiation, which vacuum chamber has an optical axis for the generated EUV radiation as it exits the vacuum chamber, with high repetition rates and high average outputs, preferably for the wavelength region of 13.5 nm.
b) Description of the Related Art
Sources for EUV radiation or soft X-ray radiation are promising radiation sources for the next generation in semiconductor lithography. Radiation sources of this kind which work in pulsed operation can generate radiation-emitting plasma in different ways based on laser excitation or on an electrically triggered gas discharge. The present invention is directed to the latter.
Structure widths between 25 and 50 nm are generated with EUV radiation (chiefly in the wavelength range of 13.5 nm). In order to achieve a sufficiently high throughput of wafers per hour in semiconductor lithography, in-band radiation outputs of 600 W to 700 W in a solid angle of 2π·sr are specified for the EUV sources to be used. “In-band” radiation output designates the spectral component of the total emitted radiation which can be processed by the imaging optics.
A characteristic variable for an EUV source is conversion efficiency, which is defined as the quotient of EUV in-band output (in 2π·sr) and the electrical power dissipated in the discharge system. It is typically around 1 to 2%. This means that electrical outputs of about 50 kW are used in the electrode system for the generation of gas discharge. This results in extremely high heating of the electrodes.
Empirical findings show that the life of the electrodes is limited by two effects:                a) electrode consumption due to the current flow (Imax≈30-50 kA, duration≈500 ns) during the discharge process. Local overheating and evaporation take place in a very thin surface layer.        b) electrode consumption due to melting and evaporation of the electrode material at high average input powers.        
The first effect a) represents a limit in principle. This effect can be reduced only by using electrode materials with the lowest sputter tendency (sputter rates) and/or by reducing the current density through selection of suitable electrode geometries. Effect b) is usually reduced by good cooling.
However, at high pulse repetition frequencies, i.e., at high repetition rates of the EUV source, another aspect must be taken into consideration.
According to effect a), the electrode surface is highly heated during an excitation pulse (see also FIG. 1). Because of the finite thickness (e.g., 5 mm) of the tungsten layer of the electrodes and the finite speed of the heat flow to the actual heatsink (the cooling time is around 10 μs depending on the material and geometry of the electrode), the next discharge already takes place before the electrode surface has reached the coolant temperature again. Therefore, the electrode surface is heated again during a series of discharges. Estimates show that the surface temperatures of the electrodes would be permanently (and not just periodically at every individual discharge) above the melting temperature for input-side pulse energies of 10 J at repetition rates of more than 5 kHz (continuous operation). In practice, this means that continuous operation of a gas discharge pumped EUV source for repetition rates of more than 5 kHz is impossible. A test for reducing electrode erosion was carried out by M. W. McGeoch. WO 01/91523 A1 describes a photon source in which a large number of particle beams are generated so as to be distributed over spherical electrode surfaces in such a way that they meet at a point referred to as the discharge zone. The ion beams generated in a vacuum chamber are accelerated toward the center of the discharge zone and partially discharged by means of concentric (cylindrical or spherical) electrode arrangements with circular openings resulting in a linear acceleration channel for every ion beam. In this way, a dense, hot plasma generating EUV radiation or soft X-ray radiation is formed in the center of the arrangement.
A disadvantage consists in that the adjustment for exact centering is complex and the plasma generated in this way is characterized by rather strong fluctuations of the center of gravity.